It is known from biomechanics that, in the natural rolling movement of the foot, between the front part of the foot and the heel, approximately in the region of the cuneiform bones, that is to say over the shank of the shoe bottom, there is a twisting action about an axis which extends substantially in the longitudinal direction of the foot. In order to take account of that twisting movement, it is known to provide the shank of the shoe bottom intentionally with weakened locations which permit the front sole portion to twist relative to the rear sole portion about an axis extending substantially in the longitudinal direction of the shoe (German patent specification No. 804 901). Such weak locations are produced by reducing the thickness of the sole or the shoe bottom in the region of the shank thereof, while desirably there still remains a central limb portion extending substantially in the longitudinal direction of the shoe (see German published specification (DE-AS) No. 14 85 804), or it is possible to provide recesses which extend inwardly from the side edge of the shoe bottom and which are filled by means of a filling material which is less stiff (German patent specification No. 943 996).
The weak locations for reducing torsional stiffness in the shank of the shoe bottom necessarily also result in a reduction in the resistance to bending of the shoe bottom about a transversely extending axis. That is also desirable, in accordance with the above-indicated teaching, in which connection it was only proposed that a wedge-like lengthwise sole should be provided to prevent the shoe bottom from wearing out in the shank region of shoes with a heel member (see German utility model No. 17 19 678), insofar as that design also provided the foot with a firm support surface in the shank region. Particularly in the case of sports shoes however it has been found that the twistability or torsional mobility, which is desired in principle, in respect of the front sole portion relative to the rear sole portion, results in inadequate guiding and holding of the foot if the shoe bottom is not only yielding in respect of torsion in the shank region but is also readily bendable in that region, because as a result the foot has an excessive freedom of movement in the region of the metatarsus joints. That makes itself noticeable as a lack of lateral stability particularly when the foot of the runner encounters surface irregularities, which is generally inevitable when rambling or hiking, when running through woods, when jogging and the like.
The invention is therefore based on the problem of providing a shoe bottom of the kind set forth in the opening part of this specification, which ensures that the foot is better guided and held, without adversely affecting the desired torsional decoupling as between the front sole portion and the rear sole portion.